为什么full join 会报错?而right(left) join没有关问题?

为什么full join 会报错?而right(left) join没问题?在线等。。。

select * from (

SELECT agency_id,username, count( authorid ) AS num,sum(replynum ) as replynum

FROM `agencypartreply`

WHERE replynum >4

AND countdate > UNIX_TIMESTAMP( ‘2012-09-01’ )

AND countdate < UNIX_TIMESTAMP( ‘2012-11-01’ )

GROUP BY username

) as a 

FULL JOIN 

agencyuser as u on u.id = a.agency_id where a.num > 21




因为mysql没有full join这个东西

你只能写成

sleect * from A left join B on A.id=B.id

union all

select * from A.right join B on A.id=B.id

为什么full join 会报错?而right(left) join没有关问题?

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